This invention is generally concerned with the purification of liquids. Of particular interest is the treatment of water which contains unacceptable amounts of dissolved salts, such as seawater, brackish water or hard water. Such waters may be purified by forcing the water through a semipermeable reverse osmosis membrane, leaving behind the contaminants or salts which do not pass through the membrane.
A reverse osmosis membrane must reject a high fraction of the dissolved salts. It is also important that such membranes pass a relatively large amount of water (i.e., have a high water flux) through the membrane at relatively low pressures. In addition, the membrane must be tolerant to chlorine, which is often added as a disinfectant, since removal of the chlorine to protect the membrane may not always be effective.
Many U.S. Patents describe membranes which are useful in desalination processes, see for example, those cited and discussed in U.S. Pat. No. 4,830,885 to Tran et al. One of the earliest patents to describe membranes of the type used in the present invention is U.S. Pat. No. 3,744,642 to Scala et al.
The semipermeable membrane used in the desalination process ordinarily will be relatively thin in order to increase the flux. Thus the membrane often is formed on a porous support to provide strength to the composite. The result often being referred to as a thin film composite (TFC) membrane. The supports should have pores which are sufficiently large so that the water (permeate) can pass through the support without reducing the flux of the entire composite. Conversely, the pores should not be so large that the thin semipermeable membrane will be unable to bridge the pores or will tend to fill up or penetrate too far into the pores. Scala et al. suggest that with pores above about 8 microns the rejection of impurities is reduced.
In U.S. Pat. No. 3,904,519, McKinney et al. disclose the use of linear polyamides which are cast on a support and then crosslinked to form a reverse osmosis membrane. The polyamides are prepared by condensation of diamines with dicarboxylic acid chlorides and crosslinked by formaldehyde, irradiation, or diamines.
In a patent unrelated to reverse osmosis membranes, but which is related to the chemistry often employed, Stephens in U.S. Pat. No. 3,920,612 discloses the reaction of aromatic primary diamines with an acyl halide derivative of trimellitic anhydride to produce a polymer containing both free carboxyl groups and amide groups which can be further reacted.
The formation of prepolymers for later reaction to make reverse osmosis membranes is discussed in the Final Report, OWRT Contract 14-34-001-6521 (March 1978) pages 7-10 and also U.S. Pat. No. 4,259,183. An improvement in salt rejection was sought by reacting piperazine (a cyclic aliphatic secondary diamine) with trimesoyl chloride, cyanuric chloride, and phosphorous oxychloride to produce a compound having amino groups which could be interfacially reacted with an acyl halide to form a reverse osmosis membrane While improved salt rejection was obtained, the water flux was considered to be low. The same report suggested that monoamines might be used, particularly morpholine and diethanol amine, but stated that tests indicated that no beneficial effect was obtained.
Such prepolymers were acknowledged by the inventor of the '183 patent in U.S. Pat. No. 4,277,344, however, monomeric starting materials were preferred.
Rak et al. disclose in U.S. Pat. No. 4,606,943 preparation of a prepolymer by reacting an aromatic diamine with an aromatic anhydride acyl halide, followed by formation of a reverse osmosis membrane by interfacially reacting the prepolymer with an acyl halide on a support. The method inherently provides pendant carboxylic acid groups in the prepolymer derived from the anhydride.
In U.S. Pat. No. 4,761,234 Toray Industries disclosed semipermeable membranes which have tri-functional aryl residues having at least two --NH groups. In their examples the patentees used compounds containing --NO.sub.2 groups and reduced them to --NH.sub.2 groups by hydrogenation over a Pt on carbon catalyst. The resulting polyamines, which contained 3 or 4 --NH.sub.2 groups were subsequently reacted with a polyacyl halide to form the desired membrane.
Kraus et al. in U.S. 4,233,434 disclosed polymers based on phosphoric acid which included reacting phosphorous oxychloride (POC1.sub.3) with a nucleophile such as an amine to remove one chlorine atom, leaving a phosphorous dichloride, which is then reacted with a nitroaromatic nucleophile (e.g., nitro aniline). The product is reduced by catalytic hydrogenation to provide a diamine containing phosphorous and that diamine is polymerized with an aromatic acid chloride (e.g., isophthaloyl chloride).
The present inventors have now discovered a new method for preparing a reverse osmosis membrane and employing a new aromatic polyamine for interfacial reaction with an acyl halide or other amine-reactive group. The resulting membrane provides a high salt rejection with good water flux and good chlorine tolerance.